For aesthetic purposes, a paint is applied to industrial products such as an automobile, home electric appliances and the like. For example, as illustrated in a drawing of FIG. 10A, a metallic painted automobile includes a steel plate substrate 60 to which an electrophoretic paint film 61 is formed in order to prevent corrosion and the like. Further, an anti-chipping primer paint film 62 is formed on the electrophoretic paint film 61 in order to prevent the automobile body from damage of a flying gravel or the like. Further, a middle coat paint film 63 is formed on the anti-chipping primer paint film 62, a base paint film 64 including a pigment and flake pigments is formed on the middle coat paint film 63, and a clear paint film 65 not including the pigment and flake pigments is formed on the base painting film 64. The electrophoretic paint film 61 is formed for corrosion prevention of the substrate, and the anti-chipping primer paint film 62 is formed for preventing the substrate from the damages caused by the flying gravel or the like. A paint film thickness of each paint films needs to be measured and properly controlled because the corrosion prevention function and the damage preventing function may be deteriorated when the paint film thickness is less than a set thickness. Further, the middle coat paint film 63, the base paint film 64 and the clear paint film 65 are closely-associated to the appearance (e.g., color, degree of metallic, brilliance, orange peel and depth of color) of the product. Accordingly, the paint film thickness of each film needs to be measured and properly controlled.
Generally, a thickness of each paint film is measured in a manner where the paint film is firstly dried and measured by means of an eddy current paint film thickness measuring device. The eddy current paint film thickness measuring device may cause damage to the product and may not measure paint film thicknesses of multiple layers.
A non-contacting type paint film thickness measuring device based on an optical interference has been developed in order to reduce the damage caused to the product, however, such device may not measure each paint film thickness of the multiple layers (JP3542346B and JP3326961B).
Further, another non-contacting type paint film thickness measuring device emitting a terahertz pulse light to an object whose paint film thickness is measured has been developed in response to the abovementioned matters (JP2004-28618A, P6-7, FIGS. 1, 5 and 6). The terahertz pulse light is an electromagnetic wave whose wavelength is 30 to 3000 μm and frequency is 0.1 to 10 THz. The terahertz pulse light passes through a paint film whose main element is a high-polymer material. When the terahertz pulse light is emitted to an object made of plural paint layers indicated in FIG. 10A, the terahertz pulse light is reflected (Fresnel reflection) on each interface IP1 through IP5, each has discontinuous refractive index, and a reflected terahertz pulse light (hereinafter referred to as a terahertz echo pulse light) is obtained. A wave form of electric field intensity in time-series of the terahertz echo pulse light is schematically indicated in the graph of FIG. 10B. The paint film thickness of each paint film is calculated by a formula (1) on the basis of a Time of Flight method using a time difference T1 2 between echo pulses P1 and P2, a time difference T2 3 between echo pulses P2 and P3, and a time difference T3 4 between echo pulses P3 and P4. The echo pulses P1, P2, P3 and P4 appear in the graph of FIG. 10B so as to be adjacent each other.Paint film thickness=(time difference×light speed)/(paint film's group refractive index)  (1)
When the paint film thickness is calculated on the basis of the time difference between the adjacent echo pulses, because an optical resolution for measuring the paint film thickness is determined on the basis of a pulse width of the echo pulse, an optical resolution “R” is expressed by the following formula (2) on the basis of the formula (1), in which the pulse width as “τ”, a paint film's group refractive index as “n”, and a light speed as “c”.R=τc/2n=TRc/n  (2)
In other words, as shown in the graph FIG. 11 indicating the terahertz echo pulse light, a minimum time difference TR=τ/2, in which adjacent signals of echo pulses are distinguishable, determines an optical resolution. Accordingly, in order to increase the optical resolution for measuring a paint film thickness, the pulse width of the terahertz echo pulse light needs to be shortened.
According to the abovementioned known paint film thickness measuring device disclosed in JP2004-28618A, a terahertz pulse light whose pulse width “τ” is 400 fs is used (see FIG. 5 in JP2004-28618A), and when an object whose refractive index “n” is 2 is measured, an optical resolution “R” is estimated to 30 μm. In other words, the known paint film thickness measuring device may not measure an object whose paint film thickness is less than 30 μm.
FIG. 11 schematically illustrates the graph indicating a terahertz echo pulse light. Practically, noise may be superposed on the terahertz echo pulse light, and the optical resolution may further be decreased.
Furthermore, the known paint film thickness measuring device has a structure where an object whose paint film thickness is measured is positioned on the optical path between a terahertz pulse light generating portion and a terahertz pulse light detecting portion in a manner where a measured surface of the object specular reflects the terahertz pulse light. Accordingly, when the measured surface of the object exists at a recessed portion of the object, the terahertz pulse light is not specular reflected on the measured surface at the recessed portion, which means a paint film thickness may not be measured when the measured surface exists at the recessed portion of the object.
A need thus exists to provide a non-contacting type paint film thickness measuring device and a method which is not susceptible to the drawback mentioned above.